1. Field of the Invention
The present invention relates to a deterioration signal generation device for an oxygen sensor which generates, from a detection signal of an oxygen sensor, a pseudo output value (degeneration output signal) simulating a deteriorated sate of the oxygen sensor, the oxygen sensor outputting a detection signal based on the concentration of oxygen in exhaust gas of an internal combustion engine.
2. Description of the Related Art
A related art oxygen sensor is known which is mounted in an exhaust passage of an internal combustion engine, such as a vehicle engine, to check whether the air-fuel ratio is rich or lean based on the concentration of oxygen in the exhaust gas. A detection signal output from the oxygen sensor is transmitted to an ECU (Electronic Control Unit) performing various engine controls, and the ECU executes feedback control of the air-fuel ratio such as by adjusting the fuel injection amount in the engine based on the detection signal thus received.
Since such oxygen sensors may slightly deteriorate due to aging upon use and exposure to exhaust gas in an exhaust gas passage over a long period of time, programs or devices are incorporated into the ECU for detecting deterioration of the oxygen sensor. While an actual vehicle is tested to check the operation of a program, etc., for detecting deterioration of the oxygen sensor, deterioration signal generation devices (deterioration simulators) capable of generating a pseudo deterioration signal simulating a deteriorated state of the oxygen sensor have been developed (see for example, Patent Document 1). For example, the deterioration signal generation device of Patent Document 1 is disposed between a normal oxygen sensor (reference sensor) mounted on a target vehicle to be tested and an ECU. The device processes a detection signal input from the reference sensor to generate a pseudo deterioration signal and outputs the deterioration signal to the ECU.    [Patent Document 1] JP-A 2004-93957
3. Problems to be Solved by the Invention
Since an ECU, however, is designed according to different standards for individual manufacturers, or vehicle models, reference potentials as a reference of the operational potential of the ECU itself may vary. Since a reference potential of an oxygen sensor connected to the ECU is subject to a reference potential of the ECU, a detection signal output from the oxygen sensor to the ECU is also subject to the reference potential of the ECU. Accordingly, increased testing accuracy has been required for each of deterioration signal generation devices outputting an appropriate signal level based on a reference potential of the ECU of individual manufacturers, or vehicle models.
In addition, the ECU is generally supplied with electric power from a vehicle battery, but arrangement of the battery and ECU of the vehicle is fixed and the wiring resistance in an electric power supply line may easily increase. In contrast, the deterioration signal generation device is readily arranged such that the wiring resistance in the electric power supply line may be relatively low as compared with that of the ECU, even though electric power is supplied from a battery or an external power supply. The reference potential of the deterioration signal generation device is adjusted to equal the reference potential of the ECU. However, for example, although each of the reference potentials is connected to ground, it is difficult to make each of the reference potentials exactly equal whenever the test is performed due to an effect such as wiring resistance.